The present invention relates to nuclear steam supply systems utilizing pressurized water as the primary coolant. More particularly, the invention relates to an improved organization for supplying high purity seal liquid at controlled temperatures to the shaft seal system of primary coolant pumps utilized in such nuclear steam supply systems.
The pumps employed in nuclear plants of the described type for circulating the primary coolant between the reactor and the steam generators therein are commonly provided wth seal element structures disposed along the pump shafts to prevent the leakage of pumped coolant therealong. It is the practice to provide flow passages in association with such seal element structures by means of which a pressurized liquid is injected into intimate relation with the seal element for cooling the elements and for preventing the accumulation of foreign deposits. Because the liquid that is injected into the shaft seal system is ultimately admitted to the primary coolant circuit and because it is necessary to prevent the inclusion of foreign deposits in the seal structure this fluid must be of high purity. It has accordingly been the practice to utilize for seal injection purposes the processed liquid returning to the primary coolant circuit from the chemical and volume control system of the plant.
As is well known, the chemical and volume control system of nuclear power plants of the described type involve a constant bleed of a portion of the circulating liquid from the primary coolant circuit of the plant. This liquid portion is diverted to chemical processing equipment for the purpose of removing corrosion and fission products therefrom and for injecting a chemical additive to maintain a proper pH level within the liquid. The system is also utilized to regulate primary coolant inventory by compensating for volumetric changes in the coolant caused by aterations in plant operation.
In most plants the chemical and volume control system includes heat exchange apparatus, termed the regenerative heat exchanger, that is effective to reduce the temperature of the diverted coolant prior to its admission to the chemical processing apparatus in order to enable the purification process to be conducted at reduced temperatures and to protect the ion exchange resin utilized therein against damage induced by excessive temperatures. In most plants this same heat exchanger is also utilized to apply heat to the processed coolant before it is reintroduced into the primary coolant circuit to maintain heat losses in the system to a minimum.
In utilizing the processed coolant from the chemical and volume control system for seal injection purposes it has been the practice to extract a portion of the processed coolant from the return line to the primary coolant circuit at a point either upstream or downstream of the regenerative heat exchanger and to divert the same to the pump shaft seal system. However, due to the fact that the coolant flowing through the chemical and volume control system is subject to thermal transients it has been necessary, in order to protect the seal structure against adverse thermal instability, to regulate the temperature of the seal injection fluid prior to injecting it into the shaft seal system. This requirement has necessitated the imposition of additional heat transfer apparatus into the system. Where the injection fluid is removed from the chemical and volume control system upstream of the regenerative heat exchanger, the additional heat transfer surface is required to raise the injection fluid temperature to a predetermined level before admitting it to the seal system and, conversely, where the injection fluid is removed from a point downstream of the regenerative heat exchanger, the added surface is needed to lower its temperature below that which would oterwise damage the seals. In either case, the need for an additional heat exchanger in such systems adds significantly to the equipment costs of the plant.